Idiot's guide to a self build battery

[Oldgreybeard]

Energy storage for us is key, our heating system relies on tonnes of warm concrete that makes up our ground floor slab, hot water heat storage is a Sunamp 9kWh phase change material thermal store, electrical energy storage is ~21kWh of batteries.

We normally only use about half the capacity of the hot water thermal store each day and much the same with the batteries, so we always have a fair bit in reserve. No idea how much of the floor slab heat storage capacity we use, my guess is less than half but it's difficult to measure.

[nowty]

Making a cuppa there as I sweat out the last of this cold and I thought to myself 'I wonder how much storage capacity we have across the membership'?

Energy storage seems to be a hard one to get in place at grid scale in significant volumes for chemical batteries. Certainly in Winter it's a critical component in the home level RE game? Then there's my personal favourite which is the low cost thermal mass batteries, concrete slabs, storage heaters etc, then super insulated hot water tanks, thermal stores, sand/wax other phase change materials. Just a thought...

I could easily add it to the secret list, its just adding a few columns and there is plenty of info already in folks Sigs to harvest. Unless you want to keep your own list.

I was even thinking about it the other week as batteries are becoming nearly as common as PV, especially with new installations. And some have even gone down the road of batteries for TOU load shifting without PV.

I would propose to have 3 categories, Batteries, Hot Water and Thermal mass.

Batteries could be gross size of any type of chemical to electrical, so Lead Acid, Lithium, Nicads, Flow, etc, etc. Most of course will be Lithium and LA. Usable size would be more accurate but I think too difficult to work out on an individual basis.

Hot water could be hot water tanks, thermal stores and phase change materials like SunAmp batteries. I think the phase change materials are too unique to have their own category, arguably they could be thermal mass but they tend to work with heating water so I think it fits better with the hot water category. I can roughly work out tank size in litres to kWh, i.e. my 300 litre tank is about 15 kWh.

Thermal mass would generally be Storage Heaters. Underfloor heated concrete slabs could also fit but probably too difficult for most folk to work out. And if the HP is running 24/7 without TOU, then its not really storage and if your running an underfloor heating HP off batteries that's double accounting in my book.

What do folk think ?

[Countrypaul]

I can see problems trying to work out how much heat is stored in an UFH slab. The same size slab could be used in different places to provide significantly differing amounts of heat, and as you suggest with top ups at different times of day. Ours is heated from a 430L Thermal store which in turn is heated from an ASHP using E7 (unless very cold) for example. What temperature would you work with, that of the UFH water in the circulation pipes (what happens with weather compensation?) or the slab temps? I would sugget the latter along with knowing the slab mass (we used 22 tonnes of S&C screed) - that combined with the low and high temps would give the heat stored - but different rooms could have different slab temps, how many know how bug there slab is?

Solvable, but I suspect only for those with detailed knowledge of their heating system (OK that might be most on this forum )

[Joeboy]

Making a cuppa there as I sweat out the last of this cold and I thought to myself 'I wonder how much storage capacity we have across the membership'?

Energy storage seems to be a hard one to get in place at grid scale in significant volumes for chemical batteries. Certainly in Winter it's a critical component in the home level RE game? Then there's my personal favourite which is the low cost thermal mass batteries, concrete slabs, storage heaters etc, then super insulated hot water tanks, thermal stores, sand/wax other phase change materials. Just a thought...

I could easily add it to the secret list, its just adding a few columns and there is plenty of info already in folks Sigs to harvest. Unless you want to keep your own list.

I was even thinking about it the other week as batteries are becoming nearly as common as PV, especially with new installations. And some have even gone down the road of batteries for TOU load shifting without PV.

I would propose to have 3 categories, Batteries, Hot Water and Thermal mass.

Batteries could be gross size of any type of chemical to electrical, so Lead Acid, Lithium, Nicads, Flow, etc, etc. Most of course will be Lithium and LA. Usable size would be more accurate but I think too difficult to work out on an individual basis.

Hot water could be hot water tanks, thermal stores and phase change materials like SunAmp batteries. I think the phase change materials are too unique to have their own category, arguably they could be thermal mass but they tend to work with heating water so I think it fits better with the hot water category. I can roughly work out tank size in litres to kWh, i.e. my 300 litre tank is about 15 kWh.

Thermal mass would generally be Storage Heaters. Underfloor heated concrete slabs could also fit but probably too difficult for most folk to work out. And if the HP is running 24/7 without TOU, then its not really storage and if your running an underfloor heating HP off batteries that's double accounting in my book.

What do folk think ?

Wouldn't even consider my own list Nowty! Would shake the pillars we are built on man! Looks good to me as definitions. We can be flexible on the numbers as some with store their hot water at 65 some at 80? I must admit to being fascinated by the concrete slabs and a long slow charge up over Summer. Just seems absolutely the way to do business.

[Oldgreybeard]

Hot water could be hot water tanks, thermal stores and phase change materials like SunAmp batteries. I think the phase change materials are too unique to have their own category, arguably they could be thermal mass but they tend to work with heating water so I think it fits better with the hot water category. I can roughly work out tank size in litres to kWh, i.e. my 300 litre tank is about 15 kWh.

The Sunamp is just another low heat loss thermal store, like any insulated hot water tank. The only differences are that they are a lot smaller and have much lower heat losses, but for comparison purposes I think it's fine to just use a "volume for volume" comparison. On that basis I think our 9kWh unit is about the same as a 210 litre hot water cylinder.

Thermal mass would generally be Storage Heaters. Underfloor heated concrete slabs could also fit but probably too difficult for most folk to work out. And if the HP is running 24/7 without TOU, then its not really storage and if your running an underfloor heating HP off batteries that's double accounting in my book.

What do folk think ?

Heat storage in concrete is easy enough to work out, as long as you know the volume (mass doesn't really matter - energy density is easier to work with, I think). 1 cubic metre of concrete stores about 2,122kJ/C°, about 590Wh/°C. Our insulated floor slab is about 8m² of concrete, so for every 1°C above room temperature it stores about 4.72kWh of useful heat (once below room temperature the heat isn't really useful any more).

[nowty]

Heat storage in concrete is easy enough to work out, as long as you know the volume. 1 cubic metre of concrete stores about 2122kJ/C°, about 590Wh/°C. Our insulated floor slab is about 8m² of concrete, so for every 1°C above room temperature it stores about 4.72kWh of useful heat (once below room temperature the heat isn't really useful any more).

And how many degrees above room temp do you heat it ?

[Oldgreybeard]

And how many degrees above room temp do you heat it ?

Varies a bit, but generally we never heat it above about 24°C and we try and keep the room temperature about 21.5°, so worst case would be about 11.8kWh. Normally the slab doesn't get warmer than about 22.5°C though, so about 4.72kWh of stored heat.

This makes it somewhat self-regulating, as the heat output from the floor drops as the room temperature rises. Heat output drops to zero when the room gets to the same temperature as the floor.

Edited to add:

If anyone wants to calculate the approximate UFH floor surface temperature to give the required heat output (in W/m²) for any given temperature differential between the floor surface temperature and room temperature, then this is a close approximation, (in Excel formula format). It's not absolutely precise, but gives an approximation that is very close (better than about 5% for the limited range of temperatures that UFH works at):

Required UFH floor surface temperature=((heat output required per unit area/8.92)^(1/1.1))+room temperature

For example, if the UFH floor area was 80m², the room temperature was 21.5°C and the heat output needed to maintain 21.5°C was 1kW then the required UFH floor surface temperature would be 22.9°C.

Most of the time we only need a few hundred watts to keep the house warm in normal winter weather. Just checked my heat loss spreadsheet and to maintain 21.5°C room temperature when it's 10°C outside needs 706W, but each of us puts out about 80W, plus there's another 250W or so of incidental heating from appliances, hot water etc losses, so with no solar gain the heating system only needs to deliver about 296W. To deliver 296W with a 21.5°C room temperature the floor surface temperature needs to be about 21.9°C. Most days there will probably be a small amount of solar gain, too, that reduces the heating power required still further.

[openspaceman]

Batteries could be gross size of any type of chemical to electrical, so Lead Acid, Lithium, Nicads, Flow, etc, etc. Most of course will be Lithium and LA. Usable size would be more accurate but I think too difficult to work out on an individual basis.

You could apply a simple rule of 50% usable capacity for LA and 90% for lithium to be a first approximation

Thermal mass would generally be Storage Heaters. Underfloor heated concrete slabs could also fit but probably too difficult for most folk to work out. And if the HP is running 24/7 without TOU, then its not really storage and if your running an underfloor heating HP off batteries that's double accounting in my book.

I would leave out thermal mass as being too difficult to quantify, my chimney breast has thermal mass I make use of and it is at 25C from last night's burn ending 17 hours ago but I cannot measure the contribution.

[Joeboy]

Batteries could be gross size of any type of chemical to electrical, so Lead Acid, Lithium, Nicads, Flow, etc, etc. Most of course will be Lithium and LA. Usable size would be more accurate but I think too difficult to work out on an individual basis.

You could apply a simple rule of 50% usable capacity for LA and 90% for lithium to be a first approximation

Thermal mass would generally be Storage Heaters. Underfloor heated concrete slabs could also fit but probably too difficult for most folk to work out. And if the HP is running 24/7 without TOU, then its not really storage and if your running an underfloor heating HP off batteries that's double accounting in my book.

I would leave out thermal mass as being too difficult to quantify, my chimney breast has thermal mass I make use of and it is at 25C from last night's burn ending 17 hours ago but I cannot measure the contribution.

No point leaving out thermal mass as it is a massive (hah) contributor to the RE life. Isn't it 'just' a density of mass value, temperature and volume and you'll get in the area of energy stored? I did it for my WBS storage brick jacket last year and it was easy enough with the path laid before me by clever men.

[Oldgreybeard]

Not sure what's really meant by thermal mass, TBH, it seems to be something often quoted but quite hard to really define. I think the key thing to know is the heat capacity, i.e how much useful heat energy is stored in any particular thing, either in Joules or Wh, perhaps. The mass is a bit irrelevant, I think, as energy density only uses volume and energy. For example, concrete stores about 2,122kJ/C° of heat per cubic metre.

You don't need to know the mass of a cubic metre of concrete to work out how much useful heat it contains, you only need to know the difference in temperature between the volume of the lump of concrete and the room temperature.

It's easy to measure or estimate the volume of something and easy to measure its temperature, so just using the energy density for common building materials allows the useful heat stored to be quickly calculated. I've listed below the energy densities for some common materials (taken from the Engineering Toolbox site). To convert these numbers to kWh/m³.°C divide them by 3600:

Brick = 1,813kJ/m³.°C

Granite = 1,896kJ/m³.°C

Concrete = 2,122kJ/m³.°C

Fireclay = 2,200kJ/m³.°C

Cast iron = 3,889kJ/m³.°C

Water = 4,190kJ/m³.°C